Although it is well accepted that bone adapts to its mechanical loading environment, the underlying mechanisms of this process have not been ascertained. It has been proposed that bone fluid flow plays a role in bone's mechanosensory system via the shear stresses that it produces on bone cells, stresses that have been shown to produce biochemical responses in bone cells in vitro. Stress-induced bone fluid flow has also been proposed to enhance mass transport in cyclically loaded bone. The activities outlined in this proposal would enable the PI to investigate a new line of research to develop novel methods to quantify microstructural fluid flow in mechanically loaded bone. Recent theoretical models developed by the PI and coworkers have proposed that in cyclically loaded bone fluid pressure gradients arc amplified around the vascular canals and that the primary relaxation of the excess bone fluid pressure occurs through this vascular porosity. In addition, in this proposal a new conceptual model is proposed to explain the enhanced molecular transport observed in recent experiments during cyclic loading. The new hypothesis proposed is that although there is no net fluid transport in the lacunar-canalicular porosity during cyclic loading, there is an enhanced convective mixing in the lacunae due to the asymmetry of the inward and outward solute fluxes arising from the molecular mixing process that occurs in each lacuna during the loading cycle. To explore this hypothesis regarding enhanced molecular transport due to an asymmetric mixing in cyclically loaded bone, the following specific aims arc proposed: (1) To develop a detailed theoretical model to describe molecular transport in mechanically loaded bone based on the assumption that net solute transport will only occur if mixing is the lacuna occurs during each loading cycle; (2) To develop novel methods to quantify this mixing process by building a mechanical loading device that will non-invasively deliver physiological loads to bone in vivo, and to refine an experimental protocol to quantify tracer transport in mechanically loaded bone; and (3) To use the developed device and protocol to explore for the first time the sequential spread of tracers in cyclically loaded bone as a function of number of cycles and distance from the vascular canal. The proposed experiments should provide new information on tile mechanisms of bone's mechanosensory system, information that could be used to design clinical devices or protocols to improve the methods of prevention and treatment of osteoporosis as well as osteopenia caused by immobilization, bed rest, or a microgravity environment, and to improve the design and functionality of orthopaedic implants such as total joint replacements and fracture fixation devices as well as biological bone tissue replacements.